Species Description:
The common snook, Centropomus
undecimalis, is a subtropical marine/estuarine species (Gilmore, et al 1978;
Shafland and Foote 1983) which is by far the most abundant and wide-ranging of
all snook species (Marshall 1958). It is easily distinguished by its sloping
forehead, protruding lower jaw, prominent lateral line, elongate body form and
the position of the tips of its pelvic fins in relation to the anus (Seaman and
Collins 1983). In C. undecimalis these do not overlap or reach the anus.

The dorsal surface is a dull gray color with a yellow to green tint, and is
separated from the ventral surface by a well defined black lateral line. The
pectoral fins, pelvic fins, second dorsal fin, and the dorsal lobe of the caudal
fin are all a bright canary yellow; however, some river specimens may be
considerably darker in color than those from coastal waters.

Other identifying characteristics
of this species: Second anal spine does not reach vertical from the caudal base;
nor do the pectoral fins reach vertical from the tip of the pelvic fins. The
maxillary reaches to or beyond the vertical as measured from the center of the
eye.

Regional Occurrence:Centropomus undecimalis ranges
from the coastal mid-Atlantic United States to Brazil, including the Gulf of
Mexico and the Caribbean, with the center of abundance being coastal Florida.
The species is restricted by water temperature to tropical and subtropical
North, Central and South America. This species was formerly thought to be
temperature limited north of Volusia County, Florida; however, C. undecimalis
has been identified as far north as Pamlico Sound North Carolina. In Florida,
the range of the common snook is sympatric with other snook species, though when
C. undecimalis does occur with other related species, it is generally
the more common.

IRL Distribution:Occurs throughout the Indian River Lagoon.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:Individuals may reach up to 140 cm fork
length (55 inches) and weigh up to 22 kg (49.5 lbs). Female specimens are
generally larger than males of the same age.

Abundance:Abundance of C.
undecimalis in Florida centers on mangrove fringed coastal waters (Gilmore
1986). Numbers of snook have declined over the last 50 years due to commercial
and recreational overharvesting and habitat degradation/destruction. A bill
passed in the Florida legislature in 1957 prohibited commercial capture and sale
of snook. Passage of this bill helped ameliorate fishing pressure on snook
populations; however habitat loss and water quality degradation may have had
more far reaching effects on snook than did commercial fishing pressures
(Gilmore, Donohoe and Cooke 1983).

Reproduction:Though Centropomus
undecimalis adults may utilize fresh water habitats, they are unable to
spawn in fresh water, as sperm become activated only in saline waters. Snook
species in Florida have been observed to congregate for spawning at the mouths
of rivers, inlets and canals. Gilmore et al. (1983) found that spawning activity
is positively correlated with monthly rainfall patterns, but not necessarily
with either temperature or salinity. However, terrestrial runoffs during rain
events directly affect phytoplanktonicprimary production via an increase in
dissolved nutrients; thus spawning activity in this species is likely related to
food availability.

When reproductively active, snook spawn in the
evening over the course of several days. In Florida, two spawning peaks are
observed: the first in June/July, the second in August/October.

Embryology:Maturation of oocytes in
follicles of C. undecimalis is typical of many teleost fishes, with rapid
water uptake (measuring approximately 200% of egg volume) associated with
cytoplasmic clarification (Wallace et al. 1993). Yolk averages 91% of egg mass
(Seaman and Collins 1983; Lau and Schafland 1982).

Larval size at hatching was approximately 1.4
- 1.5 mm SL. Larvae absorbed their yolk sacs in 4 days, upon reaching 2.2 mm SL.
Melanophores become visible at 4 mm, and adult pigmentation begins to show by 7
mm. Ossification begins by the time 5 mm is achieved, with jaws becoming
completely ossified and lined with teeth by 8.6 mm SL.

IV. PHYSICAL TOLERANCES

Temperature:Centropomus
undecimalis is eurythermal, but sensitive to cold, with lethal minimum
temperatures between 6-13 °C. In laboratory experiments, Schafland and Foote
(1983) demonstrated that snook stop feeding at 14.2 °C, lose equilibrium at 12.7
°C, and die at 12,5 °C.

Lethal maximum temperatures for snook were
shown by Chung and Mendez (1993) to be influenced by acclimation temperature. In
laboratory experiments, lethal maximum temperatures for snook were between 38.7°C and 40.7 °C.

Salinity:Snook are a euryhaline
species with a preference for mangrove-fringed estuarine habitats. However,
their wide salinity tolerance accommodates the various habitat choices made by
snook as they transition from freshwater to estuarine and marine habitat areas.

Other Physical Tolerances:Juvenile snook have been
shown to survive dissolved oxygen levels of 0.4 ppm. This is partially due to
the fact that survival of juveniles in low oxygen is weight dependent: larger
snook (>160 mm SL) die faster under hypoxic conditions than do smaller
specimens. They are, however, better able to migrate to more tolerable conditions.
Conversely, small snook are not as effective at migration, yet they compensate
for this deficiency by having higher ventilation rates than larger snook.

Under hypoxic conditions at
depth, juvenile snook have been observed to move toward more oxygenated surface
waters where they decrease their activity levels.

V. COMMUNITY ECOLOGY

Trophic Mode:C. undecimalis and
other snook species can be considered opportunistic carnivores, feeding
primarily on other pelagic fish species. The specific diet of C. undecimalis
changes with habitat type. Juveniles in freshwater habitat areas in Florida prey
primarily on palaemonid shrimp, microcrustaceans and Gambusia affinis (Mosquitofish).
In salt marshes, snook maintain a similar diet, but may add other species such
as Cyprinodon variegatus (Sheepshead Minnow). Snook in seagrass habitats
feed primarily on fish and crustaceans, with Anchoa mitchelli (Bay
Anchovy), Lagodon rhomboides (Pinfish), and penaeid shrimp composing the
bulk of the diet.

Larval and juvenile growth
patterns follow the classic pattern of slow initial growth in the month
following hatching. However, nearly tripled growth rates occur thereafter until
the end of the first year. Snook larvae may grow as much as 1 mm per day. This
rate slows to approximately 0.15 mm per day after larvae reach 2.4 mm SL.

Snook are pelagic feeders with 2
daily feeding peaks: one approximately 2 hours before sunrise, and the second
approximately 2-3 hours following sunset. The tidal cycle has been shown to
affect feeding behavior, with increases in feeding activity noted with an
increase in water flow following standing flood or ebb tides.

Competitors:Snook are considered to
be top predators with the exception of humans.

Habitat:Juvenile snook utilize 3
distinct habitat areas in their first year: freshwater tributaries, salt marshes
and seagrass beds. The smallest snook, those averaging 27 mm SL, primarily
inhabit fresh water. When these small fish reach 40 -60 mm SL they migrate to
salt marsh habitat areas, where they remain approximately 60 -90 days. Juveniles
will next migrate to seagrass beds once they attain approximately 100 mm SL, and
will remain in this habitat for 4 - 5 months. Seagrass beds 5 - 15 km from ocean
inlets are the preferred habitat areas for Florida snook over 150 mm SL.
Maturation begins when juveniles reach approximately 30cm. At this time many
juvenile snook then disperse to various fresh water, brackish and marine habitat
areas, and will remain generally non-migratory as adults except for congregating
for spawning in high salinity areas.

Fisheries Importance:
Though commercial fishing for snook is illegal in Florida, the species is still
vitally important economically. In 1986, Florida's sport fisheries for
snook, tarpon, and other game fish were estimated to be worth $5 - 7 billion
annually when all adjunct enterprises associated with sport fishing are taken
into account.

Muller et al (2001) assessed snook stocks in Florida
and estimated that the highest overall abundance of snook occurred in the
southern portion of the Indian River Lagoon. Angler survey information
shows that approximately 90% of the snook captured are released, with an average
of 35,000 snook harvested annually from the 5-county area that encompasses the
Indian River Lagoon. Most (48.3%) are captured from the shoreline to 3
miles offshore, while harvests within the Indian River Lagoon account for 36% of
the total, or an average of approximately 13,000 fish per year. Inland
waters other than the Indian River Lagoon accounted for 14.4% of the total,
while offshore captures to 200 miles offshore accounted for only 1.3% of the
total.

Figure 1. Survey data for the
common snook
recreational fishery showing the number of
fishes
harvested in East Florida waters from 1997 - 2004.

Figure 2. Summary of the
common snook
recreational harvest and percentage of total by
area from 1997 -
2004.

Table 2. By-county annual and cumulative percentages of the common snook
harvest for the years 1997 - 2001. Data provided by National
Marine Fisheries Service, Fisheries Statistics Division, NOAA.

To 3 Miles

To 200 Miles

Other Inland

IRL

No. Fish

134,922

3,579

40,266

100,564

%

48.30%

1.28%

14.42%

36.00%

Table
3. Summary of the common snook recreational harvest and
percentage of total fish captured in each area from 1997 - 2004. Data
provided by National Marine Fisheries Service, Fisheries Statistics
Division, NOAA.